Flag: crate name

[steveklabnik]

This issue is part of #125

--crate-name: controlling the name of the crate

Using this flag looks like this:

$ rustdoc src/lib.rs --crate-name mycrate

By default, rustodc assumes that the name of your crate is the same name
as the .rs file. --crate-name lets you override this assumption with
whatever name you choose.

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Should we keep this flag or not?

[steveklabnik]

This seems important to keep, and I believe we even use something like it in the code already?

[euclio]

cc #155